1. Field of Invention
The current invention relates generally to apparatuses, systems and methods of detecting explosives. More particularly, the apparatuses, systems and methods relate to detecting explosives with different frequencies, and specifically, using a Nuclear Quadrupole Resonance (NQR) measurement system/device.
2. Description of Related Art
Modern day threats to the safety of people involve detonating explosives. Therefore, a variety of ways to detect explosives have been developed. For example, scanning machines have been developed to look through luggage at an airport to allow baggage screeners to see explosives and other weapons. Other screening devices are used to screen aircraft passengers. More recently NQR detection systems have been deployed to detect and identify explosives. Explosives and other compounds have unique NQR signatures that may consist of multiple NQR frequency emissions. The ability to see more than one NQR frequency from baggage or on persons improves the probability of correctly identifying explosives. Systems that do not look for multiple NQR frequencies may produce false positive results. A need, therefore, exists for an improved means for preventing false positives in NQR systems for remotely detecting explosives.
The NQR emissions from explosive substances are at a very low amplitude level compared to the RF signal that stimulated the emission. Most NQR measurement systems utilized a pulsed RE signal to stimulate the substance that will radiate NQR emissions. The measurement system produces a strong RF pulse, which could be several hundred watts, that is transmitted through a RF probe enclosed within an RE-isolated chamber that contains the package containing the suspected explosive material. Once the pulse is transmitted, the transmitter immediately shuts down, turning on circuitry to dampen probe resonances. A receiver is then turned on to listen to the weak NQR emissions from the substance. NQR emissions from nitrogen containing materials as well as other substances that produce NQR emissions exhibit an exponential decay. The receiver, if turned on quickly enough, captures the residual decaying emissions. Depending on the decay time constants of various materials, the pulse repetition rate can be slow thus preventing or limiting longer integration times to capture the emissions with stronger Signal-to-noise ratio.
A CW NQR measurement system, can pump more RF energy into those NQR frequencies at a much lower overall power, say a watt or lower. Moreover, the CW measurement system has more time to integrate over the NQR emissions from substances. In a CW NQR system, the receiver is simultaneously on while the transmitter is illuminating the package containing the suspect material. Receiver dynamic ranges exceeding 130 dB to 150 dB are required to receive the NQR emissions in the presence of the transmitted excitation signal to prevent self-jamming. Such receivers would be pushing the state of the art. An alternative solution is to provide a method to cancel as much of the transmitted signal as possible before it can enter the receiver but still fully illuminate the suspect material. This lowers the dynamic range requirements of the receiver to standard levels to be able to receive the weak NQR emissions from the explosive materials. This invention comprises a bridge circuit through which the CW excitation signal passes, but yet the receiver, connected across the balanced terminals of the bridge is isolated from the transmitted signal. One side of the bridge connects to the NQR measurement probe while the other side is connected to an impedance machine that balances the bridge.
The ability to perform NQR measurements at human-safe power levels enables the NQR measurement system to be incorporated within an RF-isolated corridor or room or chamber through which could check a person for explosives as that persons walks through a probe. Since most NQR frequencies are below 15 MHz, the hallway would appear as a waveguide operating in cutoff whereby no external man made or atmospheric RF noise could enter while people could pass through walking. Multiple probes could be installed in the hallway to test the passerby at multiple suspected explosive material NQR frequencies.